REST API: Support 'object' and 'array' types in register_meta() schemas.

Extends meta registration to support complex schema values, mirroring the functionality in the settings controller.
Error when trying to modify a meta key containing schema-nonconformant data.

Props @TimothyBlynJacobs, @birgire, @mnelson4, @flixos90.
Fixes #43392.



git-svn-id: https://develop.svn.wordpress.org/trunk@45807 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/includes/functions.php

@@ -1,4 +1,5 @@
 <?php
+require_once __DIR__ . '/class-basic-object.php';
 
 /**
  * Retrieves PHPUnit runner version.